This application claims the priority of German patent document 10 2007 052 409.0, filed Oct. 31, 2007, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process and a system for discharging a free-floating test mass that is surrounded by an electrode casing in a satellite.
In the so-called LISA (laser interferometry space antenna) program, three satellites are positioned in space at the corners of an equilateral triangle having a side length of 5 million kilometers. Each satellite is equipped with two test masses or test cubes made of a metal alloy (Au/Pt), which are maintained in a floating state by means of a very precisely controllable electrostatic field. The test masses are charged as a result of cosmic radiation or, during the release of a mechanical holding device, are provided with an electric charge to achieve the free-floating state. To avoid interference forces which are generated in an electric field because of charging and which negatively influence the actual measurements, it is to be ensured that the test masses are discharged. Moreover, in order to maximize the time available for the actual measurement, the discharge should take place as fast as possible.
For discharging the test masses, it is known to estimate the charge of the test mass by applying voltages by via so-called control electrodes which surround the test mass. The movement of the test mass caused by the voltage application is determined, and a conclusion can be drawn from the amplitude and phase of the movement concerning the charge. The test mass and the casing surrounding the test mass including the control electrodes are exposed to ultraviolet light, with the photoelectric effect being utilized here. Finally, direct voltages are applied to the control electrodes in order to assist the charge transfer. To discharge the test mass, control commands determined for this purpose from a control computer situated on the ground are required at the control unit of the satellite. The discharging is therefore performed more or less manually, and it is therefore time-consuming and imprecise.
It is therefore an object of the present invention to provide a process and a system for discharging a free-floating test mass in a satellite, which permit more precise discharging of the test mass, in a simpler manner.
This and other objects and advantages are achieved by the process according to the invention, in which the electrode casing, enclosed by a vacuum tank, has one or more electrodes for applying electrostatic forces and/or moments to the test mass and one or more second electrodes for modulating, particularly high-frequency, alternating voltages for measuring purposes onto the test mass. By means of modulated-on alternating voltages, the electrodes can be used for the application of electrostatic forces and moments also for the electrostatic measuring of the test mass position and/or of the test mass orientation.
The electrode casing also comprises one or more light-emitting elements which expose the test mass, the electrode casing, and/or the electrodes to radiation, particularly by means of ultraviolet light, for generating a photoelectric effect. A test mass charge is determined automatically and iteratively, and a control operation is carried out for eliminating the determined test mass charge until the test mass charge has reached a defined target value.
By means of the process according to the invention, the test mass floating freely in a satellite can be automatically discharged onboard the satellite. The discharge can be performed rapidly in a time-optimized manner, so that the time available for the scientific measuring period is increased. The process as well as the system are robust with respect to uncertainties of the used hardware components (particularly the light-emitting elements) as well as with respect to additional interference effects, such as modulation voltages, which are used for electrostatic measurement and control or actuation of the test mass. In addition to rapidly discharging the test mass, the invention has the advantage that no ground-side expenditures are required. The ruggedness of the process of the rapid test mass discharge results in high reliability and precision.
The invention also includes a computer program product encoded with machine-readable program commands for a control unit of a data carrier, which cause the control unit to implement the process according to the invention. The computer program product according to the invention may be a physical medium with stored program commands, such as a semiconductor memory, a floppy disk or a CD-ROM. The computer program product may also be a non-physical medium, such as a signal transmitted by way of a computer network.
Finally, the invention also provides a system for discharging a free-floating test mass surrounded by an electrode casing of the above-described type in a satellite, including devices for implementing the process according to the invention. These have the same advantages as explained in connection with the method according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.